Laboratory Manual for Introductory Geology 4e

EXERCISE 12.2
Name:
Course:
Relative Ages in Cross-Cutting Situations (continued)
Section:
Date:
(e) The process that folded the sedimentary rocks in the bottom photo must be
What can you say about the relative ages of the sedimentary rock layers? Explain.
than the rocks.
(f) Knowing that contact metamorphism “cross-cuts” the rocks intruded by a pluton, how could you tell whether a
basalt layer between two shale beds was a lava flow or a sill?
metamorphism is another process that affects rocks without physically cutting across
them. Thus, a lava flow bakes the pre-existing rock or sediment that it flows on.
12.2.3 The Principle of Inclusions
Pieces of one rock type are sometimes included in (enclosed in) another rock.
Inclusion is most common in clastic sedimentary rocks, in which fragments of older
rocks are incorporated into conglomerates, breccias, and sandstones. It also happens
where igneous rocks intrude an area and enclose pieces of the host rock. Inclusions
of different rock types in an igneous rock are called xenoliths (from the Greek
xenos, meaning stranger, and lith, meaning rock) (FIG. 12.3). Applying our common
sense again, the principle of inclusions states that such inclusions must have existed
before the intrusion or before the sedimentary rock formed, and therefore must be
older than the intrusion or sedimentary rock that encloses them.
FIGURE 12.3 Inclusions in igneous and sedimentary rocks.
Xenoliths
5cm
(a) Gabbro xenoliths in the Baring Granite in Maine.
(b) Fragments of rhyolite tuff in conglomerate from Maine.
12.2 PHYSICAL CRITERIA FOR DETERMINING RELATIVE AGE
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